Modulation of cloned skeletal muscle sodium channels by the scorpion toxins Lqh II, Lqh III, and Lqh alpha IT

The scorpion alpha-toxins Lqh II, Lqh III, and Lqh alpha IT from Leiurus qu inquestriatus hebraeus are representatives of typical alpha-toxins, specifi c for either mammals (Lqh II) or insects (Lqh alpha IT), and alpha-like tox ins (Lqh III) which act on both mammals and insects. For a comparative stud y of the effects of these toxins on mammalian sodium channels we stably exp ressed rat skeletal muscle sodium channel alpha subunits (mu I) in HEK 293 cells and measured Na+ currents in the whole-cell patch-clamp mode. The alp ha- and alpha-like toxins strongly slowed down channel inactivation with a half-maximal effect at 1.4 nM (Lqh II), 5.4 nM (Lqh III), and 0.5 nM (Lqh a lpha IT). The recovery from fast inactivation was accelerated by all toxins with the potency sequence: Lqh II > Lqh alpha IT > Lqh III. The voltage de pendence of inactivation and recovery from inactivation were reduced while the threshold for activation was only slightly shifted by congruent to 10 m V without altering the slope factors, suggesting uncoupling of the impaired inactivation from the activation. The toxins induced an increase in peak i nward current, which was accounted for by an increased maximal open-channel probability. Although all three toxins induced similar modifications of th e channel properties, their kinetics of association and dissociation were v ery different. Between -140 and -80 mV toxin association was not voltage de pendent. In 100 nM toxin the association time constants were: 1.3 s (Lqh II ), 20 s (Lqh III), and 3.8 s (Lqh alpha IT). At positive voltages the toxin dissociated from the channel; at +100 mV the dissociation time constants w ere 30, 321, and 135 ms, respectively. In contrast to the association, diss ociation was voltage dependent with a similar slope of about 12 mV per e-fo ld change for all three toxins. The strong differences in the association a nd dissociation kinetics of these toxins may identify them as members of di fferent scorpion alpha-toxin subgroups.